1. Field of the Invention
The present invention relates to the tilting of a directivity characteristic of an antenna array, e.g., an antenna array for a radar sensor.
2. Description of the Related Art
A radar sensor can be used on a motor vehicle, for example, to determine a distance between the motor vehicle and a motor vehicle traveling in front. In other specific embodiments, other objects in the surrounding field of the first motor vehicle can also be ascertained by the radar sensor. To ensure than an object is correctly sensed, respectively detected by the radar sensor, a longitudinal axis of the radar sensor should assume a predetermined angle to the horizontal plane. If the actual angle deviates from the predetermined angle, for example, due to a loading of the motor vehicle in front of or behind the center of gravity thereof, this causes the radar sensor to transmit the radar signals too far up or too far down. This is referred to as a misaligned radar sensor.
The German Patent DE 19 857 871 C1 describes one option for mechanically adjusting a radar sensor. It provides for mounting an optical mirror on the radar sensor and for using a laser source to determine the position of the mirror relative to a longitudinal axis of the motor vehicle and, if indicated, to adapt the same.
The published German Patent Application DE 10 2007 041 511 A1 describes a method for determining an angle between a longitudinal axis of the radar sensor and a longitudinal axis of the motor vehicle that provides for aiming the radar sensor at a simulated target.
Both of these discussed methods require that the radar sensor be mechanically adjusted to align the transmitting direction thereof at the desired angle relative to the longitudinal axis of the motor vehicle.
Methods are also known for determining the angle between the longitudinal axis of the radar sensor and that of the motor vehicle without using external devices. Generally, the backscattering behavior of various objects is ascertained during operation of the radar sensor, and the angle is determined therefrom. When a position of the target object is subsequently determined, the thus ascertained angle can be taken into account, whereby the object can be successfully sensed, respectively the position thereof can be determined with improved accuracy.
In the case of transmitting frequencies used by a radar sensor, what is generally referred to as a patch antenna is customarily used. Such antennae can be realized inexpensively using a process whereby structural features are produced, in particular by etching, on a substrate having high-frequency capability. A single radiation element (patch) can emit the high frequency, for example. To achieve a more concentrated bundling of the radiated high frequency, a larger aperture and thus a narrower antenna lobe can be achieved by arraying a plurality of radiation elements. If all of the radiation elements are to be fed from the same source, this can be done in parallel via a distribution network, or they can be arrayed in series. The U.S. Patent Application Publication 2007/0279303 A1 discusses such a serial feeding, for example.
Techniques are also known for influencing the main lobe direction of an antenna array through superposition. This allows oscillations having different frequencies or different phases to be superposed in a way that modifies the beam direction of the main lobe of the antenna array in a predetermined manner. However, phase shifters for modifying the phase are expensive to manufacture and not simple to operate due to the poor temperature stability thereof. Varying the frequency can, in fact, change the main beam direction, however, each frequency has another main beam direction permanently associated therewith. Also, varying the frequency typically has only little effect on the main beam direction. Therefore, in certain circumstances, adequately tilting the beam direction requires changing the frequency to the point where it is outside of a permitted frequency band. For a radar sensor, this is particularly significant in terms of automotive long-range sensing.
It is, therefore, an object of the present invention to provide an antenna array and a transmitting device that support varying the main beam direction with little outlay.